[ars]

Why is the definition of adulthood "finish maturing"? If anything that should be middle age, i.e. in the middle of adulthood, not the start of it.

By your definition as soon as someone becomes an adult they start degrading in mental ability. The peak of mental ability should match the middle of the productive years.

But in any case the maturity of the brain is not just biologically controlled - it's environmentally controlled. The brain of someone treated like an adult will be more mature than that of someone teated like a child.

The brain responds to stimulus, it doesn't really have a pre-programmed makeup. It has basic abilities, but that ones that are expressed are the ones that it needs/uses.

[tankbot]

All fair points, but I never defined adulthood, only disagreed with the parent which stated "the cut-off is not biological, it's social". Of course there are social factors as well as biological. I'm sure I'm not qualified to form such a definition, though if I tried it would have to include many factors.

I, for example, moved out of my parents' house at 18, but I wouldn't say I became an 'adult' until at least my early 30's. Even now I feel like a kid most of the time, in spite of my many responsibilities - even with a kid of my own!

I don't think we ever finish maturing, and I think we never stop learning, even though our 'peak' mental ability may come and go.